1. Field of the Invention
The present invention relates to an output buffer which provides a signal from a first circuit to a second circuit which operates with signals at a higher voltage level than the first circuit.
2. Description of the Related Art
With microprocessors manufactured using process technology enabling transistor size to be reduced well below submicron level, the maximum source to drain, gate to drain and gate to source voltage which such transistors can tolerate is likewise reduced. For instance, with one process referred to here as a 2.5 volt circuit process, transistor gate oxide thickness is reduced so that the maximum gate to source, gate to drain, or drain to source voltage a microprocessor transistor can tolerate without failure is approximately 2.7 volts.
The reduced size of the transistors utilized in microprocessors enables an increase in operation speed as well as an increase in density of transistors which can be integrated onto a single chip. Because of the increase in speed and density of transistors, the process technology utilized in manufacturing microprocessors is also desirable for use in other circuits.
For example programmable logic devices (PLDs), such as field programmable gate arrays (FPGAs), are some times utilized to perform the function similar to a microprocessors. The desirable operation speed and transistor density for PLDs is similar to microprocessors, so it would be desirable to use the same processing technology to manufacture such PLDs. However, unlike microprocessors, PLDs are utilized as glue logic to connect to devices with an operation voltage range which transistors made using the 2.5 volt circuit process will not tolerate.
For instance, PLDs typically connect to CMOS logic devices which operate over a 0-5 volt range, as well as TTL devices which operate over a 0-3.3 volt range. Although a transistor made using the 2.5 volt circuit process can produce a TTL output high of 2.4 volts, it is desirable for such a device to produce a voltage significantly higher than the 2.4 volt legal output high to assure errors do not occur.